In the telecommunications industries, there is a demand for ever-increasing rates of data transmission. Optical communication networks are being used on an increasing scale on account of their ability to carry much higher data rates than electrical links, and various improvements are constantly being developed to allow optical data links to handle higher and higher data transmission rates. Unfortunately, the higher the data transmission rate, the shorter the maximum possible length of a data link, before regeneration of the data signal is required.
Currently, terrestrial telecommunications networks are being designed with a data rate of 2.5 Gb/s over 400 km, and 10 Gb/s over 100 to 150 km. Recently, there has been the introduction of optical amplifier components such as erbium-doped fibre amplifiers. These have allowed system designers to extend the physical length of an optical link before electrical regeneration is necessary.
Generally, the limiting factor for the length of a link is chromatic dispersion, which occurs because a transmitter has a real optical linewidth and the refractive index of a fibre varies, dependent upon wavelength. The optical fibres along which the data is transmitted have already been deployed extensively and so the system constraints are transferred into the transmitter design. For a given transmitter, the optical linewidth is determined by two factors, these being the inherent linewidth at DC and the broadening of the linewidth introduced by modulation. The latter factor is referred to as static chirp. In addition, individual components may introduce a shift to the centre-frequency of the optical linewidth; this effect is usually referred to as dynamic chirp.
The modulation of an optical signal results in optical harmonics of the modulation frequency about the carrier frequency. If this modulated light is passed through a length of fibre which exhibits chromatic dispersion (that is, the fibre refractive index varies with wavelength), the phase of the light at the distal end of a fibre varies as a function of its frequency Detection of received light causes mixing of the various frequency components, but as these will have differing phases, the mixing will result in the amplitude of the detected signal being changed on account of the linewidth of the transmitted signal.
As a result of static and dynamic chirp, there is introduced into an optically modulated and transmitted signal a pulse width change and an amplitude shift. In the case of the former, higher frequencies are reduced in amplitude and at some combinations of link length, dispersion and frequency, the signal to be detected can be nulled completely. In the case of the latter, positive dynamic chirp will broaden the width of a pulse propagating down a fibre and negative dynamic chirp will narrow the pulse Either of these effects, if sufficiently large, will render a modulated signal undetectable.
With increasing speeds of transmission, it is important that a transmitter can achieve a defined and reproducible level of dynamic chirp. In order to maximise the repeater spacing, transmitters are being specified with a single value of dynamic chirp, with a specified tolerance. There is thus a demand for a modulator able to introduce a specified level of dynamic chirp into a modulated signal.